1. Field
The present disclosure relates to solid oxide electrolytes, solid oxide fuel cells including the solid oxide electrolytes, and methods of preparing the solid oxide electrolytes.
2. Description of the Related Art
Fuel cells, which are an attractive alternative energy source, can be categorized as polymer electrolyte membrane fuel cells (“PEMFCs”), phosphoric acid fuel cells (“PAFCs”), molten carbonate fuel cells (“MCFCs”), and solid oxide fuel cells (“SOFCs”), according to a type of electrolyte used.
SOFCs electrochemically generate power in an environmentally friendly and highly efficient manner by directly converting the chemical energy of a fuel gas into electric energy. Compared to other types of fuel cells, SOFCs use relatively inexpensive materials, have a relatively high tolerance to fuel impurities, enable hybrid operation, and have high efficiency. In addition, because SOFCs can use hydrocarbon-based fuels without having to reform the fuels into hydrogen, a simple and inexpensive fuel cell system can be manufactured because an external reformer can be omitted.
An SOFC includes a negative electrode at which oxidization of, for example, hydrogen or a hydrocarbon occurs, a positive electrode at which oxygen gas is reduced into O2− ions, and a ceramic solid for conducting the O2− ions.
A commercially available SOFC operates at a high temperature, about 800 to about 1000° C., and thus high-temperature alloys or expensive ceramic materials, which are able to endure high temperature conditions, are used in the commercially available SOFC. In addition, due to the high-temperature operation, the commercially available SOFC has a long initial operating time, and the durability of materials used in manufacturing the commercially available SOFC is degraded. Furthermore, the commercially available SOFC is expensive, which is the largest obstacle for common-availability. Accordingly, much research into fuel cells operating at a temperature equal to or lower than 800° C. is being performed. The operating temperature of an SOFC is highly dependent upon the characteristics of an electrolyte used. A currently available electrolyte is an yttria-stabilized zirconia electrolyte and there are many efforts to replace the yttria-stabilized zirconia electrolyte with a doped ceria electrolyte that has high ionic conductivity even at a low temperature.
Even if the doped ceria electrolyte has high ionic conductivity, it also shows high electronic conductivity in a reducing environment, and thus, when used as a solid electrolyte of a fuel cell, a power-generation efficiency of the fuel cell may be degraded. Accordingly, there remains a need to develop a doped ceria electrolyte with improved power-generation efficiency, as well as high ionic conductivity.